Hybrid vehicle systems may be configured with various transmission components. For example, in power-split transaxle hybrid vehicle systems, the transmission may include a planetary gear system directly coupled to the engine, and further coupled to the wheels via one or more rotating components, such as gears, pinions, and bearings. During vehicle operation, transmission control takes into consideration the speed constraints of the various rotating transmission components to avoid damage to the components.
The inventors herein have recognized a potential issue with such systems. The mechanically-imposed speed constraints of the transmission components may limit the maximum attainable vehicle speed, in particular during engine-off modes of operation. Specifically, when the vehicle is operated in an electric mode with the engine off, and the vehicle is being propelled by energy from a system battery, the maximum allowable vehicle speed is limited to a speed that protects the rotating transmission components from speed-incurred damage. Based on the configuration of the transmission components, it may be possible to spin the engine using the battery to reduce the speed-incurred damage to the rotating transmission component. However, the battery usage would be increased by engine parasitic losses, such as pumping losses. Limited vehicle speed ability may degrade the operator's drive experience while increased battery usage may affect the battery's life and performance, as well as degrade the vehicle's fuel economy.
In one example, some of the above issues may be at least partly addressed by a method of operating a hybrid vehicle system comprising during an engine-off mode with only an electric motor driving the vehicle, raising vehicle speed by increasing the speed of an unfueled engine. The method further comprises adjusting a cylinder valve based on the engine speed. In this way, airflow through the spinning unfueled engine can be reduced, thereby reducing engine parasitic losses.
For example, a hybrid electric vehicle may be configured with a planetary gear transmission. During selected conditions when the vehicle is driven only by an electric motor, a controller may allow the vehicle speed to be increased up to a threshold vehicle speed without spinning the engine. As such, up to the threshold speed, as the vehicle speed increases, the rotational speed of a rotating transmission component (such as a gear component) may also increase. Above the threshold vehicle speed, further rotation of the transmission component may lead to mechanical degradation of the component. Therefore, above the threshold vehicle speed, the controller may allow the vehicle speed to be further increased by spinning the engine unfueled. Based on the configuration of the transmission component with relation to the engine and the wheels, as the engine speed is increased, the rotational speed of the transmission component may be decreased (or maintained at the upper limit), allowing the maximum vehicle speed attainable in the electric-only mode to be raised. While the engine is spinning unfueled, the position of a cam phaser may be adjusted to reduce airflow through the spinning engine. This may enable the operation of a cylinder valve (e.g., for an intake valve and/or an exhaust valve) to be adjusted. For example, a valve timing may be advanced or retarded, as appropriate, to reduce an airflow through the spinning engine. By reducing the airflow, parasitic engine losses, such as pumping losses, may be reduced, thereby allowing the vehicle to continue to be operated using the battery for a longer duration.
In this way, higher vehicle speeds may be attained while continuing to operate a hybrid vehicle in a fuel-efficient electric mode with power provided from a system battery. By adjusting a cylinder valve operation to minimize airflow through the spinning engine, engine pumping losses may be reduced. Additionally, oxygen loading of an exhaust catalyst can also be reduced. By maintaining a rotational speed of a rotating transmission component within limits, speed-induced mechanical degradation of transmission components may be reduced. By prolonging the electric mode of vehicle operation without limiting vehicle speed, the operator's drive experience can be improved while improving the fuel economy of the vehicle.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.